Web Forms :: Button Click Event Not Getting Called Due To Tag?

In my web form, i am having button named "btnSave" and there are some textbox controls (to name few, Emp Number, Full Name, etc). I've added RegexValidator to the textbox controls as below:

<asp:RegularExpressionValidator
ID="employeeNumberValidator"
ValidationExpression="^[0-9]+$"
ControlToValidate="txtEmpNumber"
ErrorMessage="Only integers are allowed"
runat="server">
</asp:RegularExpressionValidator>

This RegexValidator will check for emp number textbox contains only integers and if not, it will display message "Only integers are allowed". Now, in my btnSave_Click(...) event, i am first validating whether the textbox contains data and if not, then i am highlighting the textbox in red. When txtEmpNumber contains non-integers data and if i click the Save button, the code in inside btnSave_Click(...) event is not getting called. But when i provide employee number in expected format and then click Save button, then the code inside button click event is getting called without any issues.

I am not sure why my btnSave_Click(...) is not functioning when Employee Number has data in incorrect format.